Hand-held ignition voltage tester

ABSTRACT

A hand-held ignition voltage tester for detecting voltage on a spark plug wire in an ignition system, such as a distributorless ignition system. The tester includes a housing, positive and negative power cables and a capacitive probe for capacitively coupling to the spark plug wire. The capacitive probe generates a voltage signal which is representative of the voltage on the spark plug wire. A plurality of voltage level indicators are mounted to the housing to form a bar graph, with each indicator corresponding to a selected voltage level. A measurement circuit is coupled to the power cables and the capacitive probe for activating the voltage level indicators in response to the voltage signal.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. Ser. No. 08/729,754 which was filed onOct. 7, 1996 and issued as U.S. Pat. No. 5,834,939 on Nov. 10, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to automotive test equipment and, inparticular, to a hand-held ignition voltage tester for detecting asecondary voltage in a spark plug wire.

The majority of automobiles manufactured today use distributorlessignition systems similar to the systems that have been used onmulti-cylinder motorcycle engines. Distributorless ignition systems usemultiple ignition coils, with each coil having two secondary outputsconnected in a series with one another. The two secondary outputs areconnected to the spark plugs of a pair of companion cylinders, and bothspark plugs fire simultaneously but with opposite polarities. A fourcylinder engine uses two ignition coils and a six cylinder engine usesthree ignition coils. In a four stroke engine, one spark plug firesduring the compression stroke and the other spark plug fires during theexhaust stroke. Therefore, each spark plug fires twice every enginecycle. The spark plug firing that occurs during the compression strokeis called a “useful” firing and the firing that occurs during theexhaust stroke is called a “waste” firing. The secondary voltagerequired by a spark plug during a compression firing is much higher thanthe voltage required during an exhaust firing.

Most present day ignition voltage testers use multiple secondary probeswhich are connected to each spark plug wire. Each probe has an assignedpolarity, either positive or-negative. The operator is required to knowthe firing voltage polarity of each spark plug wire for a give engine tobe able to connect the probes to the correct spark plug wires. Thissignificantly increases the time required for the test since theoperator must first look up the firing voltage polarity for each enginetested. This also increases the level skill required to properly testthe ignition voltage.

Another disadvantage of most present day ignition voltage testers isthat they use expensive displays to display the peak ignition voltagesgraphically. These displays include a cathode ray tube (CRT) or an LCDgraphic display, for example. Also, these displays require complexcircuitry to obtain the entire secondary waveforms, measure the peakignition voltages and drive the displays, which further increases thecost of the tester. The displayed waveforms require a skilled mechanicto read and properly interpret the data.

Yet another disadvantage of most present day testers is that the probesused to pick up the secondary voltages Lack sufficient isolation. Atypical probe includes a metallic spring-loaded, alligator-type clamp.The metallic frame of the clamp itself is used as the pickup. A clamp ofthis type picks up stray signals from adjacent spark plug wires. Theclamp must therefore be moved sufficiently far away from adjacent sparkplug wires, if possible, to obtain a proper reading.

SUMMARY OF THE INVENTION

The hand-held ignition voltage tester of the present invention isinexpensive, provides an easy to read output and is simple to use inthat it requires no knowledge of spark plug polarity and resists straypick-ups from adjacent spark plug wires. The tester includes a housing,positive and negative power cables and a capacitive probe forcapacitively coupling to a spark plug wire. The capacitive probegenerates a voltage signal which is representative of the voltage on thespark plug wire. A plurality of voltage level indicators are mounted tothe housing, with each indicator corresponding to a selected voltagelevel. A measurement circuit is coupled to the power cables and thecapacitive probe for selectively activating the voltage level indicatorsin response to the voltage signal. The measurement circuit activateseach voltage level indicator for which the voltage signal exceeds thecorresponding selected voltage level.

In one embodiment, the tester further includes an absolute value circuitand a plurality of comparators. The absolute value circuit has an inputcoupled to the capacitive probe and has a magnitude output. Eachcomparator has a first input coupled to the magnitude output, a secondinput coupled to a respective reference voltage and an output coupled toa respective one of the plurality of voltage level indicators.

The voltage level indicators are preferably arranged on the housing in aline to form a bar graph. In one embodiment, the tester further includesa low voltage indicator at a low end of the bar graph which is activatedto indicate a low voltage condition when the magnitude output is lessthan a selected minimum voltage level. An open circuit indicator ispositioned at a high end of the bar graph and is operated by themeasurement circuit to indicate an open circuit condition when thevoltage signal exceeds a selected maximum voltage.

In a preferred embodiment, the capacitive probe includes a conductiveprobe handle and a clamping member for clamping to the spark plug wire.A first insulator is attached to the clamping member, which insulatesthe clamping member from the spark plug wire. A shielded cable extendsfrom the housing and is attached to the probe handle. The shielded cableincludes an inner conductor, a second insulator surrounding the innerconductor, a conductive shield surrounding the second insulator, and athird insulator surrounding the conductive shield. The probe handle isgrounded to the conductive shield. The inner conductor extends along theclamping member between the clamping member and the first insulator toform a capacitive pick up. The conductive shield is stripped from theinner conductor along the clamping member only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a distributorless ignitionsystem.

FIG. 2 is a top plan view of a hand-held ignition voltage testeraccording to the present invention.

FIGS. 3a and 3 b are schematic diagrams of the tester shown in FIG. 2.

FIG. 4 is a detailed view of a shielded cable used in the tester shownin FIG. 1.

FIG. 5 is a plan view of a capacitive probe coupled to the shieldedcable, in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a simplified schematic diagram of a distributorless ignitionsystem. Ignition system 10 includes positive terminal 12, groundterminal 14, switch 16, ignition coil 18 and spark plugs 20 and 22.Ignition coil 18 has a primary side 24 which is coupled in seriesbetween positive terminal 12 and ground terminal 14, through switch 16.Ignition coil 18 has a secondary side 26 with a pair of secondaryoutputs 28 and 30. Output 28 is coupled to spark plug 20 through sparkplug wire 29, and output 30 is coupled to spark plug 22 through sparkplug wire 31. Spark plugs 20 and 22 are coupled to ground terminal 14,through gaps 32 and 34. Spark plugs 20 and 22 are mounted within a pairof companion cylinders (not shown) and fire simultaneously with oppositepolarities. Switch 16 is opened and closes to create a change in currentin primary side 24 of ignition coil 18, which results in a largesecondary voltage developed in secondary side 26. The secondary voltageincreases until the charge developed on spark plugs 20 and 22 dischargesacross gaps 32 and 34. The secondary voltage at which spark plugs 20 and22 fire depends upon the resistance of spark plug wires 29 and 31, thewidth of gaps 32 and 34 and the condition of the spark plug electrodes.Therefore, ignition performance can be determined by observing thesecondary voltages developed on each of the spark plug wires on theengine.

FIG. 2 is a top plan view of an ignition voltage tester in accordancewith the present invention. Tester 50 includes housing 52, positivebattery cable 54, negative battery cable 56 and high tension (H.T.)probe 58. Positive battery cable 54 includes conductor 60 and clamp 62.Negative battery cable 56 includes conductor 64 and clamp 66. Positiveand negative battery cables 54 and 56 provide power to tester 50. Probe58 includes shielded cable 68 and capacitive pick-up 70. Capacitivepick-up 70 is attached to a spark plug wire, such as wire 29 shown inFIG. 1, for generating a measurement signal indicative of the voltage onthe spark plug wire. Probe 58 is discussed in greater detail below withreference to FIGS. 4 and 5.

A plurality of voltage level indicators 72 a-72 l are mounted to housing52 in a line to form a bar graph. In preferred embodiment, voltage levelindicators 72 a-72 l each include a light emitting diode (LED) whichcorresponds to a selected secondary voltage level. For example,indicators 72 b-72 k correspond to 4, 6, 8, 10, 12, 14, 16, 18, 20 and22 KV, respectively. Indicator 72 a corresponds to a voltage of lessthan 4 KV, and indicator 72 l corresponds to a voltage level of greaterthan 24 KV. Tester 50 activates each indicator 72 b-72 l for which themeasurement signal exceeds the corresponding secondary voltage level.For example, if the measurement voltage represented a secondary voltagelevel of 15 KV, indicators 72 b-72 g would be lit.

Indicator 72 a is a low-voltage indicator which lights to indicate alow-voltage condition when the measurement voltage represents asecondary voltage of less than 4 KV. Indicator 72 l is an open circuitindicator which lights when the measurement signal represents asecondary voltage that exceeds a maximum voltage level, such as 24 KV.Preferably, indicators 72 a and 72 l are a different color thanindicators 72 b-72 k. For example, indicators 72 a and 72 l may be red,while indicators 72 b-72 k may be green. The red indicators wouldindicate a failure condition.

FIGS. 3a and 3 b together form a schematic diagram of tester 50. Asshown in FIG. 3a, tester 50 includes a power supply circuit 100 and aninput circuit 102. Power supply circuit 100 includes inputs J1, J2 andJ3, diode D6, capacitors C3, C4, C5 and C6, resistor R13, voltageregulator 104, DC-to-DC converter 106 and transistor Q1. Inputs J1, J2and J3 are coupled to battery positive cable 54, battery negative cable56 and probe 58, respectively. Inputs J2 and J3 are further coupled toground terminal GNDA of tester 50. Diode D6 is a reverse voltageprotection diode having an anode coupled to input J1 and a cathodecoupled to input 108 of voltage regulator 104. Capacitor C3 is a filtercapacitor which is coupled between the cathode of diode D6 and groundterminal GNDA. Voltage regulator 104 further includes a ground terminalGND which is coupled to ground terminal GNDA and an output 108 which iscoupled to positive 9-volt supply terminal 110. Capacitor C4 is coupledbetween output 108 and ground terminal GNDA. Voltage regulator 104includes an LM2940T regulator, for example, which is available fromNational Semiconductor Corporation.

DC-to-DC converter 106 has an input 112 which is coupled to positive9-volt supply terminal 110. DC-to-DC converter 106 inverts the voltagereceived on input 112 and thus supplies −9 volts on output 114.Converter 106 further includes a ground terminal 116 which is coupled toground terminal GNDA. Capacitor C6 is coupled across inputs 118 and 120of converter 106. Converter 106 includes a National Semiconductor LT1054DC-to-DC converter, for example. Capacitor C5 is coupled between output114 and ground terminal GNDA. Resistor R13 is coupled between the baseof transistor Q1 and ground terminal GNDA. The emitter of transistor Q1is coupled to output 114, and the collector of transistor Q1 is coupledto negative 9-volt supply terminal 122. Supply terminals 110 and 122supply power to the various elements of tester 50.

Input circuit 102 includes capacitive divider 128, variable gainamplifier 132, absolute value circuit 134, measurement capacitor C2 andrange amplifier 136. Input J4 is coupled to probe 58 (shown in FIG. 1).Capacitive divider 12 and 13 formed by the capacitive pick-up of probe58 and capacitor C1, which are coupled in series, with capacitor C1being coupled between input J4 and ground terminal GNDA. Resistor R51 iscoupled in parallel with capacitor C1. Capacitor C1 and probe 58generate a voltage on input J4 that is representative the secondaryvoltage of the spark plug to which the probe is clamped. Resistor R1 iscoupled between input J4 and node 130. Protection diodes D1 and D2 arecoupled between node 130 and supply terminals 122 and 110, respectively.

Variable gain amplifier 132 includes operational amplifier U1A, variableresistor R2 and resistor R3. Resistor R2 is coupled between theinverting input of amplifier U1A and ground terminal GNDA. Thenon-inverting input of amplifier U1A is coupled to node 130. Resistor R3is coupled between the output of amplifier U1A and the inverting inputof amplifier U1A. The resistance of resistor R2 may be adjusted toadjust the gain of amplifier 132 and thereby calibrate tester 50 tochanges in the capacitance or resistance of the particular probe that isused with tester 50.

Absolute value circuit 134 includes operational amplifiers U1B and U1C,diodes D3 and D4 and resistors R4-R8. Absolute value circuit 134 has aninput 138 which is coupled to the output of amplifier U1A and amagnitude output 140 which is coupled to the anode of Schottky diode D5.Absolute value circuit 134 receives positive or negative voltages oninput 138 and supplies a positive output voltage on output 140 which isproportional to the absolute value of the magnitude of the input voltageon input 138. Diodes D3 and D4 allow only positive voltages to passthrough amplifier U1B, while both positive and negative voltages passthrough amplifier U1C. Amplifier U1B operates as inverting amplifierhaving unity gain, the output of which is added to the inverting inputof amplifier U1C. Resistor R8 has one-half of the resistance of resistorR5. Amplifier U1C therefore operates as an inverting amplifier having again of two for the output of amplifier U1B. Negative voltages areinverted through amplifier U1C with unity gain. Thus, the voltageresulting on output 14C is positive and has a unity gain for bothpositive and negative input voltages. Absolute value circuit 134 is anexample of an absolute value circuit that is useful with the presentinvention. Other absolute value circuits can also be used. In apreferred embodiment, absolute value circuit 134 has unity gain.

Schottky diode D5 is coupled between magnitude output 140 of absolutevalue circuit 134 and measurement capacitor C2. A Shottky diode ispreferred since it is relatively fast and will quickly chargemeasurement capacitor C2. The charge on measurement capacitor C2 isproportional to the secondary voltage on the spark plug to which probe58 is attached. Range amplifier 136 receives a voltage representative ofthe charge on capacitor C2 and provides an output voltage on node 142.Range amplifier 136 includes operational amplifier U1D and resistorsR10, R11 and R12. Resistor R10 is coupled between measurement capacitorC2 and the non-inverting input of amplifier U10. Resistor R11 is coupledin a feedback loop between output 142 and the inverting input ofamplifier U1D. A range selector switch 143 is coupled across resistorR11 through inputs J5 and J6. Switch 143 is normally closed such thatrange amplifier 136 has unity gain. When range switch 143 is open,resistor R11 is no longer shorted and the gain of range amplifier 136increases by a factor determined by the resistance of resistor R11. Inthis embodiment, the gain doubles. When the gain is doubled, voltagelevel indicators 72 b-72 k correspond to 4-12 KV instead of 4-22 KV.This provides increased resolution for low secondary voltages.

Input circuit 102 further includes peak storm circuit 144 which includesinputs J7 and J8, switch 146 and resistor R9. Inputs J7 and J8 arecoupled together in series with resistor R9 between the non-invertinginput of amplifier U1D and ground terminal GNDA. Switch 146 is coupledacross inputs J7 and J8. In a normal mode, switch 146 is in a closedstate such that capacitor C2 continually discharges through resistor R9.In a peak store mode, switch 146 is in an open state. The peak voltagesare no longer discharged through resistor R9 but are stored on capacitorC2. These peak voltages can then be observed on indicators 72 a-72 l(shown in FIG. 2).

Operational amplifiers U1A, U1B, U1C and U1D are preferably implementedon the same integrated circuit, such as a TL074 integrated circuitavailable from Texas Instruments Incorporated. Integrated circuit U1 isshown in the upper right hand corner of FIG. 3a and is coupled betweensupply terminals 110 and 112. A pair of bypass capacitors C11 and C12are coupled in series across integrated circuit U1. Ground terminal GNDAis coupled to a node between capacitors C11 and C12.

Referring to FIG. 3b, tester 50 further includes measurement circuit 148which is formed of a plurality of comparator circuits 150 b-150 l. Eachcomparator circuit 150 b-150 l has an operational amplifier with anon-inverting input coupled to output 142 of range amplifier 136, aninverting input coupled to a respective reference voltage and an outputcoupled to a respective voltage level indicator 72 b-72 l. In theembodiment shown in FIG. 3b, indicators 72 b-72 l include LEDs D7-D17,respectively.

For example, comparator circuit 150 b includes operational amplifier U2Aand resistors R23-R25. The non-inverting input of operational amplifierU2A is coupled to output 142 of range amplifier 136. Resistors R24 andR25 are coupled together in series between supply terminal 110 andground terminal GNDA to form a voltage divider at the inverting input ofoperational amplifier U2A. The output of operational amplifier U2A iscoupled to LED D7 through resistor R23. The voltage divider formed byresistor R24 and R25 generates a reference voltage at the invertinginput of amplifier U2A which is representative of a selected secondaryvoltage level, such as 4 KV. The reference voltage level is selected byadjusting the resistances of resistors R24 and R25. If the voltage onoutput 142 is greater than the reference voltage at the inverting inputof amplifier U2A, the output of amplifier U2A goes high, which turns onLED D7. If the voltage on output 142 is less than the reference voltageat inverting input of amplifier U2A, the output of amplifier U2A goeslow, which turns off LED D7.

Each comparator circuit 150 b-150 l has a progressively larger referencevoltage level such that LEDs D7-D17 form, a bar graph which illustratesthe instantaneous secondary voltage in the spark plug wire being tested.

Comparator circuit 150 l has the largest reference voltage level, andits associated voltage level indicator 150 l is positioned at a high endof the bar graph. The reference voltage level of comparator circuit 150l represents a selected maximum secondary voltage. If the voltage onoutput 142 exceeds the selected maximum voltage, comparator circuit 1501turns on LED D17 to indicate an open circuit condition, such as an openspark plug wire or a faulty spark plug.

Measurement circuit 100 further includes a short or low voltageindicator circuit 150 a. Low voltage indicator circuit 150 a is acomparator circuit similar to comparator circuits 150 b-150 l, but iscoupled to the output of operational amplifier U2A and has a time delay.Circuit 150 a includes diode D19, capacitor C7, resistors R47-R50,operational amplifier U4D and voltage level indicator 72 a. Diode D19 iscoupled between the output of operational amplifier U2 a and theinverting input of operational amplifier J4D. Capacitor C7 and resistorR50 are coupled together in parallel between the inverting input ofoperational amplifier U4D and ground terminal GNDA. Resistors R48 andR49 are coupled together in series between supply terminal 110 andground terminal GNDA to form a voltage divider at the non-invertinginput of operational amplifier U4D. The output of operational amplifierU4D is coupled to voltage level indicator 72 a through resistor R47.Indicator 72 a is formed by LED D18.

If the voltage on output 142 is greater than the reference voltage atthe inverting input of operational amplifier U2A (i.e., the secondaryvoltage on the spark plug wire is greater than 4 KV), the output ofoperational amplifier U2A will be high. This charges capacitor C7, whichholds the output of operational amplifier U4D low and holds LED D18 off.If the output of operational amplifier U2A goes low for a short periodof time, such as with a misfire of the ignition system, capacitor C7will remain charged holding LED D18 off. However, if the output ofoperational amplifier U2A stays low for a selected time period such as1-2 seconds (i.e., the secondary voltage on the spark plug wire is lessthan 4 KV) the capacitor C7 discharges through resistor R50 allowing LEDD18 to turn on indicating a low voltage condition. The selected timeperiod is determined by the capacitance of capacitor C7 and theresistance of resistor R50. A low voltage condition may be caused by afaulty ignition system, a shorted spark plug wire or a shorted sparkplug.

Operational amplifiers U2A-U2D, U3A-U3D and U4A-U4D are implemented inintegrated circuits U2, U3 and U4, which are shown in the upper righthand corner of FIG. 3b. Integrated circuits can include LM324N typeintegrated circuits, for example, which are available from NationalSemiconductor Corporation. Integrated circuits U2, U3 and U4 are coupledbetween supply terminal 110 and ground terminal GNDA. Bypass capacitorsC8, C9 and C10 are coupled across integrated circuits U2, U3 and U4.

The following table provides examples of component values which can beused in the circuit shown in FIGS. 3a and 3 b. These values are providedas an example only.

Element Value R1 10KΩ R2 10KΩ R3 10KΩ R4 10KΩ R5 10KΩ R6 10KΩ R7 10KΩ R85.1KΩ R9 2MΩ R10 2KΩ R11 10KΩ R12 10KΩ R13 2KΩ R14 800Ω R15 10KΩ R162.9KΩ R17 800Ω R18 10KΩ R19 2KΩ R20 800Ω R21 10KΩ R22 1.2Ω R23 800Ω R2410KΩ R25 590Ω R26 800Ω R27 10KΩ R28 8KΩ R29 800Ω R30 10KΩ R31 6.4KΩ R32800Ω R33 10KΩ R34 5KΩ R35 800Ω R36 10KΩ R37 3.8KΩ R38 800Ω R39 10KΩ R4016KΩ R41 800Ω R42 10KΩ R43 12.5KΩ R44 800Ω R45 10KΩ R46 10KΩ R47 800ΩR48 10KΩ R49 10KΩ R50 20MΩ R51 200KΩ C1 1000 pf C2 0.1 μf C3 1 μf C4 2μf C5 10 μf C6 10 μf C7 0.1 μf C8 0.1 μf C9 0.1 μf C10 0.1 μf C11 0.1 μfC12 0.1 μf

FIGS. 4 and 5 illustrate probe 58 shown in FIG. 2 in greater detail.FIG. 4 is a side plan view of shielded cable 68, which extends betweentester 50 and capacitive pickup 70 Shielded cable 68 includes innerconductor 200, inner conductor insulator 202, braided conductive shield204 and outer insulating jacket 206. As shown in FIG. 4, the distal endof cable 68 has a portion 208 at which outer jacket 206 is strippedexpose braided shield 204 and has a portion 210 at which both outerjacket 206 and braided shield 204 are stripped to expose inner conductor200 and inner conductor insulator 204.

FIG. 5 is a side plan view of cable 68 attached to capacitive pickup 70.In the embodiment shown in FIG. 5, capacitive pickup 70 includes ametallic, spring-loaded, alligator-type clamp having a pair of handles220 and 221 and a pair of clamping teeth 222 and 224, which areseparated by a pivot 226. Handles 220 and 221 are pressed together toseparate clamping teeth 222 and 224 when connecting pickup 70 to arespective spark plug wire. Rubber boots 228 and 230 are attached tosurround clamping teeth 222 and 224 for insulating teeth 222 and 224from the spark plug wire to which pickup 70 is connected. Boots 228 and230 can be attached to teeth 222 and 224 by a silicon glue, for example.

Shielded cable 68 is attached to and extends along handle 221. Handle221 includes a metallic crimp 232 which is positioned adjacent portion208 at which conductive shield 204 is exposed. Crimp 232 is crimped ontoconductive shield 204 to attach cable 68 to handle 221 and to groundhandle 221 to conductive shield 204. The distal end of cable 68 extendsalong clamping tooth 224, between the clamping tooth and rubber boot230. Outer jacket 206 and conductive shield 204 are stripped from innerconductor 200 and inner conductor insulator 210 along clamping tooth 224only. This shields inner conductor 200 along the entire length of cable68, accept within rubber boot 230.

The unshielded portion of inner conductor 200 within boot 230 forms theactual high voltage capacitive pickup. The clamp itself is grounded andforms no part of the capacitive pickup. With this structure, no straycapacitances are picked up through the clamp or through cable 68 fromadjacent spark plug wires. The clamp or the cable 68 can rest onadjacent spark plug wires without causing interference in the readingsof tester 50. This makes tester 70 much easier to use and significantlyincreases the integrity of the readings. In an alternative embodiment,the clamp includes a T-type connector, as opposed to an alligator-typeconnector.

The ignition voltage tester of the present invention provides alow-cost, easy to use approach for making secondary voltage measurementsin an ignition system. The tester can be used with distributorless orconventional ignition systems. In a distributorless ignition system, theoperator is not required to know the polarity of each spark plug beforetaking the reading. A positive voltage reading is made regardless of thepolarity of the spark plug. The LED display provides a low cost, simplevisual display of the readings which identifies the peak secondaryvoltage and identifies short circuit and open circuit conditions. Theentire tester requires a small number of very inexpensive components,which reduces the cost of the tester.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A hand-held ignition voltage tester for detectingvoltage on a spark plug wire in a distributorless ignition system, thetester comprising: a housing; an input terminal for receiving a voltagesignal representative of the voltage on the spark plug wire; a pluralityof light emitting diodes (LEDs) mounted to the housing, each LEDcorresponding to a selected voltage level; an input circuit comprisingan input coupled to the input terminal to receive the voltage signal, ameasurement output and an absolute value circuit coupled between theinput and the measurement output; and a measurement circuit coupledbetween the measurement output and the LEDs for selectively lighting theLEDs in response to the measurement output.
 2. The hand-held ignitionvoltage tester of claim 1 wherein the plurality of LEDs are mounted tothe housing in ascending order of the corresponding voltage levels toform a bar graph.
 3. The hand-held ignition voltage tester of claim 2and further comprising: an open circuit indicator positioned at a highend of the bar graph and being operated by the measurement circuit toindicate an open circuit condition when the voltage signal exceeds aselected maximum voltage; and a short circuit indicator positioned at alow end of the bar graph and being operated by the measurement circuitto indicate a shorted condition when the voltage signal is below aselected minimum voltage.
 4. The hand-held ignition voltage tester ofclaim 1 wherein: the measurement circuit comprises a plurality ofcomparators, with each comparator having a first input coupled to themeasurement output, a second input coupled to a respective referencevoltage terminal, and an output coupled to a respective one of theplurality of LEDs.
 5. The hand-held ignition voltage tester of claim 4wherein the input circuit further comprises: a variable gain amplifiercoupled between the input of the input circuit and the absolute valuecircuit.
 6. The hand-held ignition voltage tester of claim 4 wherein theabsolute value circuit has an output and the input circuit furthercomprises: a measurement capacitor coupled to the output of the absolutevalue circuit; and a discharge resistance coupled in parallel with themeasurement capacitor.
 7. The hand-held ignition voltage tester of claim6 wherein the input circuit further comprises: a peak store switchcoupled in series with the discharge resistance for selectivelydisconnecting the discharge resistance from the measurement capacitance.8. The hand-held ignition voltage tester of claim 4 wherein the absolutevalue circuit has an output and the input circuit further comprises: arange amplifier having an input coupled to the output of the absolutevalue circuit and an output coupled to the first inputs of the pluralityof comparators.
 9. The hand-held ignition voltage tester of claim 8wherein the range amplifier comprises: an operational amplifier havingan inverting input, a non-inverting input coupled to the output of theabsolute value circuit, and an output coupled to the first inputs of theplurality of comparators; a feedback resistor coupled between theinverting input and the output of the operational amplifier; and a rangeswitch coupled in parallel with the feedback resistor.
 10. The hand-heldignition voltage tester of claim 4 wherein the plurality of LEDscomprises a first LED corresponding to a selected minimum voltage leveland wherein the measurement circuit further comprises: a diode having ananode coupled to the output of the comparator that is coupled to thefirst LED and having a cathode; a reference voltage generator; anamplifier having a non-inverting input coupled to the reference voltagegenerator, an inverting input coupled to the cathode and an output; anda low voltage indicating LED coupled to the output of the amplifier. 11.The hand-held ignition voltage tester of claim 10 wherein themeasurement circuit further comprises: a time delay capacitor coupled tothe cathode of the diode and having a capacitance; a load resistorcoupled in parallel with the time delay capacitor and having aresistance; and wherein the amplifier turns the low voltage indicatingLED off when the first LED is on and turns the low voltage indicatingLED on after the first LED has been off for a selected time determinedby the capacitance of the time delay capacitor and the resistance of theload resistor.
 12. The hand-held ignition voltage tester of claim 1 andfurther comprising a capacitive probe coupled to the input terminal,wherein the capacitive probe comprises: a conductive probe handle; aclamping member for clamping to the spark plug wire; a first insulatorattached to the clamping member which insulates the clamping member fromthe spark plug wire; and a shielded cable attached to the probe handleand comprising an inner conductor, a second insulator surrounding theinner conductor, a conductive shield surrounding the second insulator,and a third insulator surrounding the conductive shield, wherein theprobe handle is grounded to the conductive shield and wherein the innerconductor extends along the clamping member between the clamping memberand the first insulator and forms a capacitive pickup, with theconductive shield being stripped from the inner conductor along theclamping member only.
 13. The hand-held ignition voltage tester of claim12 and further comprising: positive and negative power cables; a groundterminal coupled to the negative power cable and to the conductiveshield; and a voltage regulator coupled to the positive and negativepower cables and having positive and negative supply terminals coupledto the measurement circuit.
 14. A hand-held ignition voltage tester fordetecting voltage on a spark plug wire in a distributorless ignitionsystem, the tester comprising: a housing; an input terminal forreceiving a voltage signal representative of the voltage on the sparkplug wire; a plurality of voltage level indicators mounted to thehousing to form a bar graph, each indicator corresponding to a selectedvoltage level; an absolute value circuit having an input coupled to theinput terminal for receiving the voltage signal and having a magnitudeoutput; and measurement means coupled to the magnitude output and theplurality of voltage level indicators for activating each voltage levelindicator for which the magnitude output exceeds the correspondingselected voltage level.
 15. The hand-held ignition voltage tester ofclaim 14 and further comprising: a low voltage indicator; and whereinone of the selected voltage levels is a minimum voltage level andwherein the measurement means activates the low voltage indicator whenthe magnitude output is less than the minimum voltage level.